Modem communications systems increasingly rely on digital modulation techniques for reliable system performance. These techniques modulate a carrier signal by varying its phase and/or amplitude.
The heterodyne architecture is standard for radio transmitters. It uses rectangular (in-phase and quadrature-phase) data and I/Q modulation to generate the transmit signal. This architecture handles phase and amplitude modulation equally well, but also produces spurious mixing products, delivers only moderate linearity, and suffers from poor efficiency.
A more efficient radio transmitter architecture is referred to as a polar modulator. This architecture eliminates several circuits, avoids mixing operations, and improves efficiency. It also introduces a new challenge, separate amplitude and phase control. The separate control is complicated by some modulation formats that null the envelope of the transmit signal. It would therefore be advantageous to reduce the amplitude modulation while still preserving the quality of the transmit signal.